For those who aren't familiar with the early medieval idea of "sky sailors", or the account that you are asking about, it may help to give the original story you refer to and put it in some sort of context before going on to actually answer your question.

Abogard (c.779-840) was Archbishop of Lyon during the reign of Louis the Pious, who was the son of Charlemagne. This was a period in which the Christianisation of the Frankish kingdoms had been ongoing for more than three centuries, but Abogard remained concerned that some peasants retained non-Christian superstitions and beliefs, and some of his writings were devoted to efforts to counter these beliefs. The one that we're concerned with is Contra Insulam Vulgi Opinionem de Grandine et Tonitruis, or "Against Absurd Notions of the Common People on Hail and Thunder," which was written some time after 810, and – as Jean-Louis Brodu points out – very probably after 829, when an ecclesiastical council meeting in Paris denounced the belief that sorcerers could "disturb the air, send hail, remove the fruit and the milk from their rightful owners and give them to others, and make numerous prodigies."

In this treatise, Abogard complained of travelling through a district in which

almost all persons, noble and plebeian, townsmen and rustics, old and young, believe that hail and thunder may be produced at the will of man – that is, by the incantations of certain men called tempestarii ... We have seen and heard many who are sunk in such folly and stupidity, as to believe and assert that there is a certain country which they call Magonia, whence ships come in the clouds for the purpose of carrying back the corn which is beaten off by the hail and storms; and which aerial sailors purchase of the said tempestarii.

So Agobard is telling us that the people of this region believed in a "country in the sky", Magonia. These people seem to be in league with earthbound sorcerers – weather wizards – who are capable of raising storms that knock down peasant crops. Once the crops have been destroyed, Magonians appear and purchase them from the wizards.

Exactly what Agobard himself believed about all this is of some relevance. He lived at a time when large-scale battles were being fought, sometimes fairly literally, over competing interpretations of the core Christian texts – the dispute between those who approved of the veneration of religious images, and the iconoclasts who opposed the practice, was coming close to tearing Byzantium apart during this period – and his modern biographer, Allen Cabaniss, points out that he "was an opponent of the reactionary elements of the time, the judicial ordeal, weather magic, relic worship, pilgrimages, the excessive veneration of the saints, the use of images, Biblical obscurantism, [and] unrestrained ritualistic aberrations." The flip side of all this, however, was that Agobard was also a sincere Christian. The Bible makes no mention of any such place as Magonia, and so Agobard had no compunction in suggesting that belief in such a place was the product of nothing but superstition. On the other hand, there was Biblical authority – as we will see – for the idea that God had created waters of heaven as well as creating waters of earth, and it seems far less certain that he would have denied that such regions actually existed.

Agobard's account is especially interesting for the apparent eyewitness account it offers of an encounter with this belief:

We have even seen several of these senseless fools who, believing in the reality of such absurd things, brought in front of an assembly of men four persons in chains, three men and one woman, who they said had fallen from these ships. They retained them in irons for some days, before they brought them before me, followed by the crowd, to have them stoned to death as they had been condemned, but after a long discussion, the truth finally triumphed after the many reasonings which I opposed to them, and those who had shown them to the people were found, as a proverb has it, 'as much confused as a thief when he is surprised.'"

Brodu makes the important point that Agobard is not simply concerned to attack peasant "superstition" in his account of the Magonians; he is part of a broader ecclesiastical discourse, prominent in this period, designed to "suppress any form of pagan activities" and which "had already acted against much more visible magical methods of defence against hail." This effort certainly dated as far back and the 780s, when Charlemagne had prohibited the peasant practice of attempting to guard against hailstorms by planting large sticks covered with pieces of parchment in the fields. It has been suggested, quite plausibly in my view, that these parchments would have been covered with magical symbols of some sort. The Archbishop was also concerned to prevent what he saw as a form of racketeering which was going on to the detriment of the local peasantry – if the hail-damaged crops he referred to were not being sold to the Magonians, then presumably they were being traded to someone else.

Brodu comments usefully on possible meanings of the word "Magonia". He cites Jean-Claude Bologne, in his Du Flambeau au Bûcher: Magie et Superstition au Moyan Age (1993), as suggesting the word may refer to the Minorcan port of Port-Mahon, which was known during the Roman period as Portus Magonis, or Magona, and was supposed, in legend, to have been founded by Hannibal's brother Mago, whose family was known as the Magonides. Bologne notes that the inscription "Magonianus" meant "from Port-Mahon" in this period. This seems to make it not entirely certain that Magonia itself was conceived of as a "country" located in the celestial sphere. It may be that the superstition Abogard encountered was that the inhabitants of Minorca had acquired the ability to navigate sky-ships.

The beliefs that Agobard describes seem to have been pretty widespread in this period and although he is the only writer that we know of to refer explicitly to ‘Magonia,’ the earliest accounts of this sort were certainly not invented by him; in fact, they can be traced in material from early medieval Ireland that Meyer has concluded were “entirely based on oral information obtained in Ireland itself.” Very similar stories were later told in England, and continued to appear as late at 18th century Canada and even in 1890s Texas (the last being what was probably an example of the wide-eyed newspaper hoaxes common in this period). At least one of these accounts, and probably several of them, dates to before Agobard and the first half of the ninth century, and there are traces of what appear to be the same set of beliefs in sources that originated in earlier periods still.

The first ‘sky sailor’ accounts that have come down to us are Irish, and are found in the Annals of the Four Masters for 743 and in the Annals of Ulster for 749. Versions of the same account(s) can also be found in the annals of Tigernach and Clonmacnoise and in two recensions of the Lebar Gabála (“Book of Invasions”), which was compiled in the 11th century. It’s important to point out that the first two of these annals, which seem to preserve the earliest versions of the basic story, have survived only in, respectively, a compilation dating to the 1630s, and an MS dating to the late 15th century. However, scholars of Irish historical writing seem satisfied that the Ulster version, at least, contains material that was written contemporaneously from around the 550s, and as such it is probably reasonable to presume that the sky-ship story it contains predates Agobard’s.